1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
In maskless lithography it is known to project the patterned beam onto a target portion of the substrate using a projection system that comprises an array of lenses arranged such that each lens receives and focuses a respective portion of the patterned beam. Each lens of the array thus projects a respective spot of radiation onto the substrate, and the array of lenses collectively projects a radiation pattern on the substrate. Such systems are generally referred to as microlens array or MLA systems. In these systems, the patterned beam is typically projected onto the lens array through a beam expander, which comprises a series of lens components and is arranged to provide a substantially parallel radiation beam.
Ideally, the beam patterning and beam projection components should be precisely aligned (i.e., arranged with respect to each other and the target substrate), such that each spot projected onto the target substrate by a lens of the MLA is sharply focused and corresponds to a respective one of the controllable elements. To achieve this, the components should be aligned, such that the projection of the patterned beam on the MLA is in register with the MLA. Each lens of the MLA should receive and focus a portion of the beam that corresponds to a respective one of the controllable elements.
It will be appreciated that the “corresponding respective element” is the element on which the intensity of the portion of beam is primarily dependent. In general, the beam portion received by a particular lens of the MLA will have an intensity that is also dependent on the states of elements adjacent to the “corresponding” element.
Achieving precise alignment of a number of beam patterning and beam projection components to provide the desired correspondence between the projected radiations spots and the controllable elements may pose a problem.
Another possible problem is that projection beam supplied to the patterning means (i.e., the array of controllable elements) may not be uniform (i.e., it may not have a uniform intensity over its cross section). In such cases, even though the patterning and projection components may be set up with precise alignment, the radiation pattern projected onto the substrate may have an unwanted intensity modulation.
Therefore, what is needed is a lithographic system and method that allow for accurate correspondence between the projected radiations spots and the controllable elements and/or a projection beam having a uniform intensity.